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Colonies of turtle ants behave as if they are playing a game of Risk. They spread out their forces to control more resources, but also retreat if their position is not defensible.

“They’re sensitive to changes in the environment. They can change the allocation of their defenses in response to that,” says at Harvey Mudd College in California.

is a species of that lives in trees in the Sonoran Desert of Mexico and Arizona. Each colony can hold a few hundred ants, spread out over several nests in tree cavities.

The nests are defended by soldiers, which are bigger than workers and have bumpy heads that they use to block the nest entrance. But there aren’t always enough soldiers to go around. So Donaldson-Matasci and her colleagues wanted to see how the ants allocate their soldiers.

A risky game

They tracked wild nests and found that larger nests were less likely to survive, though they lasted a little longer if they had more soldiers.

In the lab, they offered a colony a choice of new nests in large or small holes. Sometimes a competing ant species was also present, creating a high-risk situation.

In the low-risk situation, the ants did not differentiate much between small, defensible nests and larger ones that need more soldiers. They also allocated more soldiers to the large nests.

But when enemies were lurking, the ants started fewer new nests. They also put proportionately fewer soldiers in the large, indefensible nests.

“We think it’s [a] risk-limiting strategy in a way,” says Donaldson-Matasci.

However, the ants still started large nests even in high-risk situations, suggesting they still took some chances. “You want to keep your fingers in enough pies,” says Donaldson-Matasci. “It’s sort of a balancing act in that sense.”

Crucially, turtle ants do not have five-star generals directing them. Instead, individual ants visit different nests and decide whether to stay or go.

“Social insects can approximate really good colony-level solutions just though local individual decision making,” says Donaldson-Matasci.